The process of frequency conversion in a nonlinear material generates heat within the nonlinear material medium due to absorption. This heat must be removed if the frequency converter is to operate efficiently at a significant power.
One method of heat removal in solid state crystalline materials employed in laser systems is to remove the heat from the sides of the materials, in a direction transverse to the direction of laser energy propagation. The removal of heat in a transverse direction causes thermal gradients in this direction. This creates two problems. The first problem is that thermal-optical stress and index variations cause thermal aberrations that distort the laser beam. The second problem is that, in most frequency conversion materials, for example, the temperature variation in a direction transverse to the direction of propagation of the laser beam must be maintained to within a very small tolerance range. The presence of a thermal gradient in this direction severely limits the aperture size and the power loading allowed in a laser system design.
U.S. Pat. No. 5,363,391, entitled "Conductive Face-Cooled Laser Crystal", and issued to Steven C. Matthews et al on Nov. 8, 1994, discloses and claims techniques for passively removing heat from an optical element in a laser system through its optically transmissive faces. Heat is removed by way of optically transmissive heat conducting media disposed adjacent the optically transmissive surfaces of the optical element. Heat is transferred out of the optical element in a direction parallel to the direction of propagation of optical radiation, thus minimizing problems associated with thermal gradients. Devices employing optical elements such as nonlinear frequency conversion crystals and laser crystals may utilize the heat management approach to achieve better performance. Heat is transferred to the heat conducting media by direct contact or through narrow gas-filled gaps disposed between the optical element and the heat conducting media.
While that patent is well-suited for its intended use, improvements are sought to overcome certain remaining problems. Specifically, that patent teaches the use of a traditional dispersive material as a face-cooling medium. However, when two or more crystals are used for efficient second harmonic generation (SHG), for example, the dispersive medium causes the fundamental and the second harmonic beams to be dephased (out of phase with respect to each other) at the output of the face-cooling medium. If a second crystal is placed next to the face-cooling medium, the random phase could cause conversion from the second harmonic back into the fundamental, decreasing the effectiveness of the SHG process. This problem was overcome on earlier multi-crystal testbeds by using the dispersive nature of air to rephase the fundamental and second harmonic. This approach, however, requires separating the face-cooled crystal modules by an air path that is different for each individual product, requiring space (many centimeters of additional beam path) and adding to the manufacturing complexity (active adjustment of crystal spacing).
Thus, there is a need to provide a face-cooling method such as taught in U.S. Pat. No. 5,363,391, but for use with multiple nonlinear crystal formats used primarily for second harmonic generation without the need for air-path rephasing between the crystals.